### Wieliczka Salt Mine
Poland — Miners spent 700 years transforming an evaporated prehistoric sea into a massive underground city carved entirely from salt.
About 13.5 million years ago during the Miocene epoch, a shallow sea covering what is now Poland completely dried up, leaving behind a massive crust of salt. Millions of years later, violent orogenic (mountain-building) tectonic movements smashed the region's plates together. The immense pressure folded and shattered this ancient seabed, trapping huge blocks of pure green salt in thick layers of underground clay.
For over 700 years, humans burrowed down to get it. Using hand tools, water-spray dissolution, and eventually modern machines, miners hollowed out over 9.4 million cubic meters of rock. They didn't just dig a quarry; they built an underground world. Stretching 327 meters deep, the mine features 2,328 colossal chambers, complete with brilliant chandeliers, statues, and entire chapels sculpted right out of the solid gray rock-salt.
But hollowing out a mountain comes with a catch. Under extreme pressure, salt undergoes "convergence"—a slow, plastic deformation where the solid walls literally warp and flow. Since a town of 20,000 people sits right above the mine, preventing a massive sinkhole is crucial. Today, geologists fiercely debate the best way to measure the shifting walls, arguing whether high-tech 3D Terrestrial Laser Scanning or highly sensitive traditional wire extensometers are best to keep this subterranean masterpiece from caving in.